Import class from vue file in typescript file

I'm using Vue 2 with a TypeScript layer (vue-property-decorator).

I have a class called BaseForm in a .vue file:

<template>
  <div class="base-form">
    <form>
      <slot />
    </form>
  </div>
</template>

<script lang="ts">
import { Component, Vue } from 'vue-property-decorator';

@Component({})
export default class BaseForm extends Vue {
  comment = '';
}
</script>

I want to access the comment variable of the BaseForm class in a .ts file like so:

import BaseForm from '@/components/BaseForm.vue';
setComment(form: BaseForm, comment: string) {
  form.comment = comment;
}

But in the .ts file the BaseForm is not typed as class but as (Vue) interface and I can not access the variable.

My shims-vue.d.ts looks like (I got that from here):

declare module '*.vue' {
  import Vue from 'vue';
  export default Vue;
}

I tried to add something like this (but it did not work):

declare module 'BaseForm.vue' {
  import BaseForm from '@/components/base/BaseForm.vue';
  export default BaseForm;
}

Any advice how to use a class in a non-vue file?